(Massaliski, 1990) credits the discovery of this structure to (Rengade, 1909), but we have been unable to obtain a copy of this reference. (Downs, 2003) quotes (Wyckoff, 1963) giving an $Immm-oI8$ structure for CsO. Since this is the same space group and Pearson symbol as found in Massaliski we use Wyckoff's data.
In the absence of the original article, we provide the ICSD link to the work of (Föppl, 1957).
M. E. Rengade, , Compt. Rend. 148, 1199–1202 (1909).
H. Föppl, Die Kristallstrukturen der Alkaliperoxyde, Z. Anorganische und Allgemeine Chemie 291, 12–50 (1957), doi:10.1002/zaac.19572910104.
Found in
T. B. Massalski, H. Okamoto, P. R. Subramanian, and L. Kacprzak, eds., Binary Alloy Phase Diagrams}, vol. 2 (ASM International, Materials Park, Ohio, USA, 1990), 2$^{nd$ edn. Cd-Ce to Hf-Rb.
R. T. Downs and M. Hall-Wallace, The American Mineralogist Crystal Structure Database, Am. Mineral. 88, 247–250 (2003).
R. W. G. Wyckoff, Crystal Structures, vol. 1 (Interscience Publishers, New York, 1963), second edn.
First cited in
D. Hicks, M.J. Mehl, M. Esters, C. Oses, O. Levy, G.L.W. Hart, C. Toher, and S. Curtarolo, The AFLOW Library of Crystallographic Prototypes: Part 3, Comp. Mat. Sci. 199, 110450 (2021). (doi=10.1016/j.commatsci.2021.110450)